1. Field of the Invention
The present invention relates to a radiation imaging apparatus that detects radiation which has passed through an object, and generates a radiation image.
2. Description of the Related Art
In recent years, a flat panel type of radiation imaging apparatus of an same size optical system using photoelectric conversion element has been in widespread use, for the purposes of enhancement of resolutions, reduction in size of housings, and suppression of distortions of images, in the field of a digital radiation imaging apparatus (X-ray imaging apparatus). The imaging apparatuses using the photoelectric conversion elements include amorphous silicon type, charge-coupled device (CCD) type, complementary metal oxide semiconductor (CMOS) type, and the like.
(1) Amorphous Silicon Type
An advantage of the amorphous silicon type of imaging apparatuses includes the ability to easily create an image sensor of a large-screen which uses amorphous silicon semiconductor on a glass substrate. However, since semiconductor characteristics are not adequate, it is difficult to perform high-speed operations. Further, amorphous silicon has more difficulty in microfabrication of semiconductor substrate on the glass substrate, in comparison to single-crystal silicon semiconductor substrate, and as a result, capacitance of output signal lines becomes larger, and kTC noise (thermal noise on capacitors) becomes more likely to be generated.
(2) CCD Type
Advantages of the CCD type of imaging apparatus include the ability of becoming fully-depleted and a high-sensitivity to radiation. However, it is unfit to trend toward a larger screen size. Since the CCD is of an electric charge transfer type, the CCD has a large area and the number of transfer stages of electric charge transfer is increased. In this case, driving voltages differ between different locations at drive end and near the center, and thus a complete transfer becomes difficult. Further, a power consumption is represented by CVf2 (C denotes a capacitance between substrate and well, V denotes a pulse amplitude, f denotes a pulse frequency). C and V tend to become large in large detector areas, thus power consumption becomes up to ten times greater in a CCD image sensor as compared to a CMOS image sensor.
(3) CMOS Type
Advantages of the CMOS type of imaging apparatus include the fact that a higher speed readout than the amorphous silicon type is made possible by microfabrication, and besides a high sensitivity can be obtained. Further, there are no problems with the number of transfer stages of electric charge transfer and power consumption as encountered in the CCD image sensor. Accordingly, it is easy to produce a large area imaging apparatus. In particular, it is well known that a CMOS type imaging apparatus offers superior performance as a moving image capturing device of large-area flat panel sensor.
Japanese Patent Application Open-Laid No. 2002-344809 discusses a technology to realize a large-area detector by using a CMOS image sensor for the photoelectric conversion element. In this case, a large-area flat panel type of radiation flat panel detector is obtained by subjecting a rectangular semiconductor substrate to a tiling process. The rectangular semiconductor substrate is obtained by cutting out CMOS photoelectric conversion element in a rectangular shape from silicon semiconductor wafer.
However, the flat panel type of radiation detector, which has used the CMOS image sensor, has amplifiers on a per-pixel basis, and thus thermal noise due to heat generation of the amplifiers has become a problem. Since images generated by the image sensor are negatively influenced by heat, image quality of the acquired X-ray images may be degraded due to the thermal noise.
In addition, due a trend towards a reduction in size and thinning of radiation imaging apparatuses, electronic components thereof must be closely packaged. For example, a distance from a control unit including electrical components for driving the radiation flat panel detector becomes nearer, thus the heat generated by the control unit may be easily transmitted to the radiation flat panel detector.
Japanese Patent Application Open-Laid No. 2009-085630, as measure to reduce thermal noise, proposes transmitting the heat generated by the radiation flat panel detector to the outside, thereby suppressing temperature rise of the radiation flat panel detector.
Japanese Patent Application Open-Laid No. 2003-194951, as measure to reduce heat of the control unit, also discloses a technique for dissipating the heat generated from the control unit to an external housing.
Despite of the state of the art described above, even if the heat of the radiation flat panel detector has been dissipated to the external housing by using the technology discussed in Japanese Patent Application Open-Laid No. 2009-085630, when the heat generated from the control unit including electronic components is conveyed to the radiation flat panel detector, it may result in adverse effects on the captured radiation images to be generated.
A technology for mitigating the heat emanating from the control unit is discussed in Japanese Patent Application Open-Laid No. 2003-194951. However, the heat generated from a power source apparatus and other electronics that control an imaging apparatus tend to add up and negatively affect imaging performance therein. Thus, the countermeasures discussed in Japanese Patent Application Open-Laid No. 2003-194951 and in the above-discussed references have not achieved an optimal solution to address the problems of heat affecting radiation flat panel detectors.
In particular, local temperature rise may occur in the electronic components, and there is a possibility that local noises be generated in the radiation imaging images previously created. The radiation images may be used in medical diagnosis, and generation of the local noises must be avoided as far as possible.